Brand differentiation.I don't understand why these manufacturers slap their UI on top of Android. I also don't understand why Android One stayed limited to the budget phones.
If all of them will sport a standard no cruft Android UI, it is hard to differentiate yourself on Hardware specs alone, as one of your competitors match 19 to almost complete 20 specs of your phone.[ Many tried that standard Android UI and burnt their hand]
For people buying offline, the UI is first chance to attract customers to phone among sea of other phones wanting the same attention.
Lastly it helps product segmentation, which business people crave: create your own problems and provide solution to it each iteration.